The long-term objective of this study focuses on the relationship between neurogenesis and brain development during vocal development in young zebra finches. Only young male and not female zebra finches learn to sing a close copy of their father's song. Like song behavior, the neural substrate for song is highly sexually dimorphic. The brain regions controlling song learning and production (vocal control regions, VCR) are approximately 5 times larger in adult males than females. However, at the onset of song learning, the VCR of both young males and females are approximately equal in neuron number, but diverge greatly over the course of vocal development by the addition of new neurons to the male VCR and cell death in female song-control nuclei. Until now, no one has tested whether cellular proliferation contributes directly to the development of the sexually dimorphic VCR in songbirds. Short- term thymidine administration will be used to characterize the incidence of cellular proliferation in the proliferative epithelium of juvenile and adult birds of both sexes. It is possible that the incidence of mitotic activity is higher in young males than in young females leading to the production of more new neurons in the brains of juvenile males. This surplus of new neurons could then become incorporated into the functional circuits involved in vocal learning and behavior thus heightening the ability of young males to learn vocal behavior. The studies proposed in this grant are aimed at characterizing neurogenesis in the zebra finch telencephalon and the relationship between neuronal production and vocal learning. These experiments are important in understanding basic processes of brain development and the neural mechanisms of vocal learning.